Geochemical prospecting



Jan. 7, 1941. G. s. BAYs 2,228,223

GEocEMIcAL PnosPEc'rING Filed Nov. 14, 1959 2 Sheets-Sheet 2 6 m m D 5B f M T U 5 u y w I m 4 m m M M .W T m M A00. ..1

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SAMPLE NUMBER INVENTOR v Geovfge5.fys El@ fm2@ 'ATToRNEY Patented Jan. 7, 1941 UNITED STATES GEOCHEMICAL PROSPECTING George S. Bays, Tulsa,

Okla., assignor to Stanolind Oil & Gas Company, Tulsa, Okla., va corporation of Delaware Application November 14, 1939, Serial No. 304,373

19 Claims.

This invention relates to geochemical prospecting and more particularly to prospecting or surveying to determine the possible presence of deepseated gas and oil deposits by determinations which measure an electrical parameter of certain chemical characteristics of the surface soils. It has been found that there is a more or less definite correlation between certain chemical characteristics of surface soils and valuable mineral deposits located far below those soils. This applies not only to correlations revealing the presence of buried ore deposits, but extends to correlations between these surface characteristics and sub-surface hydrocarbon deposits, such as oil sands. Thus for instance, the presence of various hydrocarbons, notably hydrocarbon gases, in the surface soils is some indication of the presence of petroleum deposits directly beneath or beneath and somewhat horizontally offset from the region of high hydrocarbon content. It also appears that in some instances there is a definite correlation between the mineral content of the soil and the presence of hydrocarbon gases and at least in some instances a correlation between such mineral contents of surface soils and the presence or absence of deep-seated petroleum deposits. One theory which has been proposed to account for this is that gases migrating from the deep-seated deposits carry with them entrained aqueous solutions of mineral salts which are deposited in the surface soils and that while such deposition is extremely slow it becomes substantial over the course of geological ages. Whatever the correctness of this and other theories may be, it does appear that there is definite evidence of correlation between certain functions of the mineral content of the surface soils and deep-seated oil and gas deposits.

It is an object of my invention to provide a method of geochemical prospecting involving the measurement of a function or parameter of the mineral content of surface soils. It is a more particular object of my invention to provide methods and apparatus whereby the mineral contents of surface soils can be measured rapidly and accurately. A further object of my invention is to provide a method and apparatus for measuring a function of the amount and nature of mineral salts in surface soils while the soil undergoing measurement remains in place. It is also an object of my invention to provide improved methods and apparatus for measuring electrical resistivity or conductivity of soils. Il Other and more detailed objects, advantages and uses of my invention will become apparent as the description of my invention proceeds.

In the course of this description reference will be made to the accompanying drawings which form apart of this specification and in which:

Figure A1 is a diagrammatic showing of apparatus in accordance with my invention;

Figure 2 is an alternative diagrammatic showing of apparatus in accordance with my invention;

Figure 3 is a detailed elevation partly in section showing a portion of one preferred embodiment of apparatus in accordance with my invention;

Figure 4 is a plan view looking up from the bottom of Figure 3;

Figure 5 illustrates the eect of increasing amounts of water on conductivity as measured with my apparatus;

Figure 6 shows a correlation between the conductivity of the soil as measured in accordance with my invention and its content of watersoluble mineral salts; and

Figure '7 shows a similar comparison between conductivities in the same soil measured by other methods and the water-soluble mineral salt contents of those samples.

In brief I have found that the measurement of the electrical conductivity or resistivity of a wet sample of surface soil in place in the ground constitutes an excellent geochemical prospecting tool and a good method of soil analysis.

I have previously referred and will hereinafter refer to surface soils but it is to be understood that this refers to any Weathered formation whether located just at the surface, a few inches or a few feet below the surface, or in unusual cases, as much as several hundred feet below it. In practice I prefer to` determine the conductivity or resistivity of the soil at a depth of from six inches to twenty-five feet, for instance three to ve feet, since soils very near the surface are subject to some contamination. Thus in practicing my invention a hole is preferably dug with a post hole auger or any other type of shallow drilling equipment and the conductivity or resistivity of the soil at the bottom of the hole is determined without removing the sample, the characteristics of which are to be determined. Not only is this highly convenient as compared with the removal and manipulation of a sample but I also iind that it gives far more significant results as will hereinafter appear.

This measurement\ of conductivity or its reciprocal, resistivity, is not significant for my pur- Cil poses unless the measurement is made in wet soil. Soils normally contain very variable amounts of water and their conductivities or resistivities are functions of the amount of water present aswell as the amount of mineral salts present and, in fact, the water content is normally a much more important factor than the mineral salt content. Thus, before measuring I wet or saturate the soil with water or aqueous solution while the soil is .in place in the ground. This can be done by merely passing water into the hole referred to and waiting until the: water seeps into the soil and then placing two electrodes in the wet soil at the bottom of the hole and measuring the resistivity or conductivity.

For instance, referring to Figure 1, water can be placed in a shallow bore hole I0, a probing apparatus II can be placed in the hole equipped with two electrodes I2 and I3, a potential can be applied to these electrodes by means of an A. C. or D. C. generator I4 through bridge I5 and the conductivity or resistivity can be measured by means of this bridge. However, I have found that this method does not give results which are readily reproducible since in many instances the water appears to seep out horizontally or pass through small fissures or cracks in the earth so that the soil is not uniformly water saturated between electrodes I2 and I3. Moreover, this is true to a considerable extent even if the electrodes are relatively small in vertical dimension so that they do not penetrate far into the soil. 1

In order to overcome this diillculty I nd it highly desirable to inject the Water directly between the two electrodes I2 and I3. As shown in Figure l, this is done by a small tubing I6 terminating between or adjacent to the electrodes. Water is supplied to this tubing by means of pump I'I. The water thus injected is preferably distilled water although water with some mineral content can be used. However, if water other than distilled water is used, the mineral content of the water should be reasonably constant for any given series of tests so that the measurements obtained will be comparable.

It might be mentioned in this connection that it is unnecessary to measure the specific resistivity or to do anything other than measure relative resistivities or, what amounts to the same thing, relative conductivties at spaced points over a given area to be surveyed. Thus for instance, the determinationscan be made along a survey line every one-quarter or one-tenth of a mile. Measurements can be taken on any desired basis so long as they are comparable.

Thus for instance, referring to Figure 2, a battery I8 giving a fairly constant potential can be connected in circuit with the two electrodes I2 and I3 and the current can be measuredby ammeter I9 at each of a series spaced curved survey points (switches 20 and 2| being open). These ammeter readings can be used directly as measures of the conductivity for any given series of determinations. Alternatively instead of an ammeter, a voltmeter 22 can be used by closing switch 20. Switch 2| can be closed and ammeter I9 can be dispensed with, thus using voltmeter 22 to give the potential drop across the electrodes I2 and I3. For a given series of readings these voltmeter readings will give a measure of the resistivity or conductivity from point to point. Preferably, however, b"oth ammeter I9 and voltmeter 22 are used.' In this case switch 2U is closed and switch 2l is left open (unless it is desired to pass less than all the current through ammeter I9), rheostat 23 is adjusted t0 give a constant current as measured on ammeter I3 and the voltage drop is measured by voltmeter 22 which can suitably be of the mlllivoltmeter type. i

Figures 3 and 4 show in more detail one embodiment of my electrode apparatus which I have found particularly desirable. Two spike electrodes I2 and I3 are located close together and the ends of these electrodes are pointed or bullet shaped. Adjacent these electrodes, as shown in Figure 4, is the end of the water injection tubing I6 which may suitably be made of 1A, inch steel tubing. The upper ends of the electrodes are insulated by fiber insulation 25 so that only the inch length of electrode below this insulation is actually used in the conductivity measurement. Thus it is assured that when the electrodes are thrust into the ground reproducible results will be obtained so long as there is at least 'M3 inch penetration in each instance.

'I'he electrodes I2 and I3 and the water injection tube I6 both passthrough a, hard rubber plug 26. This plug is externally threaded to correspond with internal threads 21 at the bottom of a fiber cylinder 28 which in the form used by me is 81/2 inches long. The threaded plug 26, containing holes to accommodate the electrodes and water injection tubing, screws into the bottom of fiber cylinder 28 which is rendered watertight by a washer 29. At the top of the two electrodes I2 and I3 are'contacts 30 or bindingr posts to which are attached insulated wires 3l extending up through the device and thence to the above-ground apparatus shown in Figure l or Figure 2.

Fiber cylinder 28 is internally threaded at its upper end as well as at its lower end and receives a correspondingly threaded brass connection 32. The upper end of this brass connection is internally threaded and receives an externally threaded pipe 33, for instance a -foot length of 1-inch pipe. The water injection pipe I6 extends above this pipe 33 and passes to hydraulic pump I'I of Figures 1 and 2 or to any other convenient means for forcing water under pressure into the soil between the two electrodes.

The use of the apparatus is very simple. All that is necessary. is to drill a. series of shallow holes at selected points which can be-arranged along a line or in any given pattern over an area to be surveyed, thrust the electrodes into the soil at the bottom of each hole, inject the water, apply a source of potential and measure the conductivity or any electrical parameter related to the conductivity. Alternatively the conductivity or resistivity of the soil at the surface can be measured without drilling holes but this is not preferred since the top soil is subject to contamination and is often not typical of the weathered region.

For purposes of convenience I will speak hereafter in the specification and in the appended claims of the measurement of conductivity but it will be understood that this wordfls broad enough to apply to the measurement of resistivity (which is the mere reciprocal of conductivity), to the measurement of current flow, the measurement of potential drop or to any other measurement which gives relative values which are functions of the relative conductivties.

I have found that the relative conductivties of a series of samplesas measured in accordance with my invention are particularly well correlatable with the mineral contents of those samples. However, in order to obtain the best correlation, it is important to make a series of conductivity measurements at each point. These measurements are made after injecting various amounts of water or continuous conductivity measurements can be made during water injection. As water is added the conductivity rises due in all probability to increased leaching out of the mineral salts from the soil. However, a point is soon reached at which the conductivity begins to decrease due to dilution of the electrolyte by increasing amounts of water. In order to obtain the best correlations the significant reading is the maximum conductivity.

This effect of increasing water injection is shown by Figure in which the conductivity measured in mhos (reciprocal ohms) multiplied by 106 is plotted against the amount of water injected measured in centimeters. It will be noted that the maximum conductivity is reached with a relatively small amount of water but this amount will, of course, vary with the size and spacing of the electrodes as well as with the characteristics of the soil. In any event, water is injected until this maximum is passed and the maximum reading is taken as the significant one for the purpose of correlating the results from a number of points in a survey.

To illustrate the correlation between conductivity measured with the soil sample in place in the ground as above described and the watersoluble mineral contents of those soils, I have shown in Figure 6 the results obtained with one series of samples taken near Tulsa, Oklahoma along a survey line with surveying stations onequarter mile apart. A total of eight samples were taken. In each case a hole was drilled with a hand auger to a depth of three feet, a sample of the soil in the bottom of the hole taken for subsequent analysis and the conductivity of the soil was run in place while forcing distilled water into the soil at the bottom of the hole. Readings were taken at denite intervals and only the highest conductivity reading accepted as the true value. In the laboratory the water-soluble mineral content of each sample was measured. Also a mixture was made of 100 grams of each soil sample and 250 cubic centimeters of distilled water. The conductivity of the slurry was then determined. The mixture was allowed to stand overnight, the clear liquid was decanted and the conductivity of the decanted portion was determined. The solution was then filtered and the conductivity of the filtered portion was measured. Because of the high colloidal content of the soil samples it was necessary to filter each one under pressure of 120 pounds per square inch of compressed air.

As seen in Figure 6, the conductivities (line 35) measured in accordance with my invention (i. e. with the samples in place in the ground) correlate very nicely with the total Water-soluble mineral salt content (line 36) as determined by laboratory analysis of the soil samples taken just above the bottom of the holes.

This is in marked contrast with the results shown in Figure 7. In Figure 7, line 36 is the water-soluble mineral salt content in parts per million by weight and corresponds with the same line in Figure 6. The dotted line 31 in Figure 7 represents the conductivitles of the soil slurries above described. The dashed line 38 shows the conductivitles of the decanted portions of these slurries and the full line 39 indicates the conductivities of the filtered portions of these slurries. Not only are these values much lower than those measured with the soil in place (due perhaps to the greater dilution) but they will be seen to have little or no correlation with the total water-soluble mineral salts content.

While I have described my invention in connection with certain specific embodiments thereof, these are by way of example only and my invention is not to be limited thereto but only to the subject matter of the appended claims. In the claims I refer to the measurement of electrical conductivity but this will be understood to include the measurement of resistivity or any lother value which is a function of electrical conductivity.

I claim:

l. A method of geochemical prospecting comprising wetting a soil sample while in place in the ground, inserting a pair of closely spaced electrodes in said wetted soil sample while in place in the ground and measuring by means of said electrodes the conductivity of said wetted soil sample while in place in the ground.

2. A method of geochemical prospectingcomprising saturating a soil sample with water while in place in the ground and measuring the electrical conductivity of the saturated soil sample.

3. A method of analysis for the mineral content of soils comprising saturating a soil sample in place in the ground with water and measuring the electrical conductivity of saidsaturated soil sample while in place in the ground.

4. A method of analysis for the mineral content Of soil comprising substantially completely saturating a small sample of soil in place in the ground with water, inserting a pair of closely spaced electrodes in said substantially completely saturated small sample of soil and measuring by the use of said electrodes the electrical conductivity of said substantially completely saturated small sample of soil while in place in the ground.

5. A method of geochemical prospecting comprising the measurement of soil conductivitles at points spaced over the terrain -to be surveyed comprising saturating soils with water at each of said points and measuring the electrical conductivities of said saturated soils for comparison with each other.

6. A method of geochemical prospecting comprising saturating a portion of the soil in place in the ground at each of a plurality of survey stations spaced over the terrain to be surveyed and measuring the electrical conductivity of each of said saturated soils while in place in the ground .at each of said plurality of survey points.

7. A method of geochemical prospecting comprising drilling shallow holes into the surface soil at each of a plurality of points at spaced intervals, wetting the soil at the bottom of each of said holes and measuring the electrical conductivitles of said wetted soils at the bottoms of said holes.

8. A method of geochemical prospecting comprising drilling holes into the surrace soil to a depth of from about six inches to about 25 feet of said holes and measuring the electrical conductivities of said saturated soils at the bottoms of said holes.

10. A method of soil analysis comprising inserting a pair of closely spaced electrodes into a soil sample in place in the ground, injecting water into the soil sample between said spaced electrodes to wet substantially the whole of said soil sample and measuring theelectrical conductivity of the wetted soil sample between said spaced electrodes.

11. A method of geochemical prospecting comprising inserting a pair of spaced electrodes into the soil in place in the ground, injecting Water gradually into the soil between said electrodes and measuring the electrical conductivities of the soil between said electrodes for.a`piurality of quantities of water injected.

12. A method of geochemical prospecting comprising inserting a pair of spaced electrodes into the soil in place in the ground, injecting water gradually into the soil between said electrodes and measuring the electrical conductivities of the soil between said electrodes during the gradual injection of said water.

13. A method of geochemical prospecting comprising the determination of the conductivity of the soil in place in the ground at each of a plurality of points spaced over the terrain to be vsurveyed comprising in the case of each of said points placing a pair of spaced electrodes in the soil, injecting water into the soil between said electrodes, determining theelectrical conductivities of said soil for a plurality of amounts of Water so injected including the maximum conductivity during such injection for each of said samples and comparing the relative values of said maximum conductivities for each of said survey points.

14. Apparatus for soil analysis comprising a pair of closely spaced electrodes adapted to be inserted into the soil and means for injecting a liquid into the soil in the vicinity of said electrodes to wet substantially the whole of the soil disposed between said electrodes.

15. Apparatus for geochemical prospecting comprising a pair of closely spaced electrodes adapted to be inserted into the soil and means for injecting a liquid into the soil between said electrodes to wet substantially the whole of the soil disposed between said electrodes.

16. Apparatus for geochemical prospecting comprising a pair of closely spaced electrodes adapted to be inserted in the soil, means for injecting water into the soil between said electrodes to wet substantially the whole of the soil disposed between said electrodes and means for determining the conductivity of the soil between said electrodes.

17. Apparatus for geochemical prospecting comprising a pair of closely spaced electrodes adapted to be inserted in the soil, a water injection tube terminating near said electrodes, a common carrier for said electrodes and said water injection tube and means for determining the conductivity of the soil between said electrodes. 18. Apparatus for geochemical prospecting comprising a pipe, a pair of electrodes carried by the bottom of said pipe and insulated from each other except at their lower ends and a water injection tubing carried by said pipe and terminating near the uninsulated portion of said electrodes.

19. A method of geochemical prospecting` comprising the measurement of soil conductivities at points spaced over the terrain to be surveyed comprising inserting two closely spaced electrodes intothe soil at each of said points, both of said electrodes being inserted into the same localized portion of the soil, wetting substantially the Whole of said localized portion of the soil in which said pair of closely spaced electrodes are inserted and measuring, by the use of said electrodes, the electrical conductivities of the wetted soils at each of said points for comparison with each other.

GEORGE S. BAYS. 

